In recent years, the technique relating to a large-scale integrated circuit device (LSI) has made a remarkable improvement in the degree of integration Following such improvement in the degree of integration and due to an increase in size of an IC chip, the heat value generated in a package carrying such an IC chip, has also been increased. Accordingly, the heat radiation property of a substrate material employed for a package for a semiconductor device etc. has been regarded as important. The heat conductivity of an alumina sintered body, which has generally been widely used for making IC substrates, is rather insufficient to provide the required heat radiation property. Thus, it is difficult for the alumina sintered body to cope with the increased heat value of the IC chip. To this end, a study has been made to see whether beryllia can be substituted for alumina, since beryllia has a high heat conductivity. However, beryllia is hard to handle because of its toxicity.
On the other hand, a sintered body of aluminum nitride (AlN), which has a high heat conductivity and high insulation ability with no toxicity, is considered with interest as a material for a circuit substrate or a package for a semiconductor device. In general, an impurity content of such an AlN sintered body is so reduced that an AlN sintered body with excellent heat conductivity and an electrical insulation ability is obtained. Such a sintered body also has an excellent translucense, and hence the same is preferably used for making optical filters etc. in practice. However, it is problematic to use the AlN sintered body for making a package etc. since the transmission of ultraviolet rays leads to malfunction of the IC.
To this end, colored AlN sintered bodies have been studied and several successful examples have been reported.
For example, Japanese Patent Laying-Open No. 63-233079 discloses a black aluminum nitride sintered body and a method of preparing the same. According to this literature, a black AlN sintered body can be obtained by adding a specific amount of calcium tungstate powder and/or calcium molybdate powder to AlN powder, mixing and molding the same, and thereafter firing the mixture in a non-oxidizing atmosphere.
On the other hand, Japanese Patent Laying-Open No. 63-310772 describes that a black AlN sintered body can be obtained by sintering AlN powder with metal molybdenum or a molybdenum compound, serving as an additive. However, the aluminum nitride sintered bodies disclosed in the aforementioned two gazettes have merely low heat conductivity values of about 100 W/m.multidot.K.
AlN easily incorporates an impurity such as oxygen. Thus, AlN powder of high purity is generally employed in order to improve the heat conductivity. If the aforementioned substance is added for coloring, the heat or thermal conductivity may be reduced. Thus, it has been impossible to obtain a colored AlN sintered body having a high heat conductivity.
However, a substrate material for carrying an IC chip having a high output, must have high heat conductivity, so that its heat radiation property is improved in order to prevent the IC from malfunctioning caused by a temperature rise of the IC chip. At the same time, the substrate must be colored in order to prevent the IC from a malfunction caused by the transmission of ultraviolet rays. According to the conventional technique, however, it has been difficult to provide a ceramic substrate, which can simultaneously satisfy the two requirements for the substrate material for an IC chip, as hereinabove described.